The present invention relates to a motor driving apparatus for performing variable-speed driving of a motor by using an inverter.
An induction-motor driving apparatus for performing variable-speed driving by using an inverter control apparatus is used for energy-saving operation of appliances such as fan and pump and the variable-speed driving of machines. A method which is commonly used as the countermeasure at the time of a failure of the inverter control apparatus is a method of providing a bypass circuit to a commercial-use power-supply. During the operation of the commercial-use power-supply, however, the induction motor is cut off from the inverter, then being re-injected into the commercial-use power-supply. As a result, there has existed a drawback that it is impossible to perform the variable-speed driving of the induction motor. Also, as the countermeasure, there exists a method of providing a standby-purpose inverter control apparatus in addition to the operation-purpose inverter control apparatus. At the time of the failure of the inverter control apparatus, however, the induction motor needs to be re-operated by the standby-purpose inverter control apparatus on standby after the induction motor has halted. As a result, there has existed a drawback that the system halts at one time temporarily. On account of this, in some of large-capacity inverters, a method is employed in which the control system is duplexed, and when the control system fails, the failed control system is instantaneously switched to the sound control system. This method is employed in order to shorten the switching time at the time of the failure occurrence. Nevertheless, this method has found it impossible to address a case where a main circuit system of the inverters fails. In addition thereto, in some of systems where a plurality of inverter control apparatuses are provided, the following method is employed: With respect to the plurality of inverter control apparatuses, a single standby-oriented-system inverter control apparatus is provided in its operation state. As a result of this provision, when the inverter control apparatus in operation fails, the inverter control apparatus can be switched to the standby-oriented-system inverter control apparatus on standby without halting the system.
In this way, when a failure of the inverter or an instantaneous power-failure of the power-supply occurs during operation of the induction motor, and when re-start of the operation is to be carried out, if excitation remains in the induction motor, it is necessary to synchronize phases of voltages between the inverter and the induction motor. On account of this necessity, the following control method is carried out at the time of reactivating the inverter: The control over the induction motor is restarted after rotation speed of the induction motor and speed command value of the inverter are caused to coincide with each other.